When do you need EDM?

Modern Machine Shop, Feb, 1995 by Harry C. Moser

EDM stands for electrical discharge machining but it might as well stand for Exact Difficult Machining because the applications best suited for this metal removal process are those characterized by extremely exacting tolerances and situations that would be extremely difficult or impossible to handle with any other method of machining.

Electrical discharge machining (EDM) has been a growing force in North American tool, die and mold making shops since the 1950s. During 1994, the sale of wire and ram (or diesinker) electrical discharge machines (EDMs) represented seven percent of total machine tool sales dollars in the United States.

Ninety percent of EDMs in the United States are used to produce molds, stamping dies, extrusion dies, forging dies, tool fixtures and gauges. Other applications include aircraft part production and medical parts.

Unlike our European and Asian counterparts, Noah American manufacturers do not use EDM as much for short production runs and prototyping. In fact, only 40 percent of the EDMs bought by U.S. shops have automatic wire threading. By comparison, nearly 100 percent of the wire EDMs purchased by European and Japanese shops have this option.

Today, we know these machines can open new manufacturing opportunities, while relaxing design constraints for savvy manufacturing and design engineers. That's why EDM might very well stand for the solution to Exact, Difficult Machining problems.

An Overview Of EDM

The origin of electrical discharge machining goes back to 1770, when English scientist Joseph Priestly discovered the erosive effect of electrical discharges. In 1943, Soviet scientists B. Lazarenko and N. Lazarenko had the idea of exploiting the destructive effect of an electrical discharge and developing a controlled process for machining materials that are conductors of electricity.

With that idea, the EDM process was born. The Lazarenkos perfected the electrical discharge process, which consisted of a succession of discharges made to take place between two conductors separated from each other by a film of non-conducting liquid, called a dielectric. The Lazarenkos achieved a form of immortality with this circuit, which today bears their name. Today, many EDMs use an advanced version of the Lazarenko circuit.

How It Works

During the EDM process, a series of non-stationary, timed electrical pulses remove material from a workpiece. The electrode and the workpiece are held by the machine tool, which also contains the dielectric. A power supply controls the timing and intensity of the electrical charges and the movement of the electrode in relation to the workpiece.

At the spot where the electric field is strongest, a discharge is initiated. Under the effect of this field, electrons and positive free ions are accelerated to high velocities and rapidly form an ionized channel that conducts electricity. At this stage current can flow and the spark forms between the electrode and workpiece, causing a great number of collisions between the particles.

During this process a bubble of gas develops and its pressure rises very steadily until a plasma zone is formed. The plasma zone quickly reaches very high temperatures, in the region of 8,000 to 12,000 [degrees] Centigrade, due to the effect of the ever-increasing number of collisions. This causes instantaneous local melting of a certain amount of the material at the surface of the two conductors.

When the current is cut off, the sudden reduction in temperature causes the bubble to implode, which projects the melted material away from the workpiece, leaving a tiny crater. The eroded material then resolidifies in the dielectric in the form of small spheres and is removed by the dielectric.

Growth of EDM

EDM has rapidly earned its place alongside milling and grinding equipment as a proactive, mainstream technology. EDM is best known for its ability to machine complex shapes in very hard metals. The most common use of EDM is machining dies, tools and molds made of hardened steel, tungsten carbide, high-speed steel and other workpiece materials that are difficult to machine by "traditional" methods.

The process has also solved a number of problems related to the machining of "exotic" materials such as Hastelloy, Nitralloy, Waspaloy and Nimonic, which are used on a large scale in the aeronautical and aerospace industries.

Because of technical advances in electrode wear, accuracies and surface speed, EDM has replaced many of the traditional processes in some applications. Another factor contributing to the growing use of EDM is the expansion of the work envelope, particularly when it comes to heights and tapers. Standard wire EDMs can cut parts 16 inches tall with a straightness of [ or -]0.0005 inch per side!

With the reduction in electrode wear and increased sophistication of EDM controls in rams, new EDM processes use simple-shaped electrodes to 3D mill complex shapes. EDM also is being used for polishing small, intricate surfaces.

Since EDM does not involve workpiece/tool forces like a mill or grinder, it is possible to EDM shapes that would break conventional cutting tools or be broken by them.